A wavelength division multiplexing (WDM) communication method of signal-synthesizing a plurality of light signals having wavelengths different from each other and packing them into a light signal path is recently practically used for an optical communication system in order to efficiently transmit light signals.
Therefore, it is necessary to accurately measure wavelength characteristics of optical members such as an optical fiber, optical amplifier, and optical branching unit serving as measuring objects built in an optical communication system.
FIG. 9 is a block diagram showing a configuration of a conventional wavelength characteristic measuring apparatus for measuring wavelength characteristics of the above optical members.
That is, as shown in FIG. 9, a light “a” comprising a laser beam emitted from a wavelength variable light source 1 inputs a measuring object 2 comprising optical members such as the above-described optical fiber, optical amplifier, and optical branching unit.
Moreover, a light b output from the measuring object 2 enters an optical power meter 3.
The optical power meter 3 measures the light intensity of the input light b and transmits the light intensity to a measurement control section 4 as a measured value c.
FIG. 10 is a block diagram showing a schematic configuration of the wavelength variable light source 1 in FIG. 9.
That is, as shown in FIG. 10, a light al having various wavelengths λ distributed in a predetermined wavelength region output from one face of a semiconductor laser 5 enters a diffraction grating 6 and is divided into a plurality of lights having wavelengths different from each other.
A light a2 having a specific wavelength among the lights divided by the diffraction grating 6 is reflected from a reflecting mirror 7 and enters the diffraction grating 6 again.
The reflecting mirror 7 is rotatably supported by a fixed axis 9 through an arm 8.
Moreover, the arm 8 is rotation-controlled by a motor 10.
The motor 10 is rotation-controlled by a motor driving circuit 11.
Therefore, by rotating the motor 10 by the motor driving circuit 11, it is possible to control the attitude angle from the diffraction grating 6 of the reflecting mirror 7 and the distance between the reflecting mirror 7 and the diffraction grating 6.
This makes it possible to guide the light having a designated specific wavelength λ2 designated among lights diffracted by the diffraction grating 6 into the diffraction grating 6 again and return the light to the semiconductor laser 5.
Moreover, the distance from the reflecting mirror 7 up to the semiconductor laser 5 through the diffraction grating 6 changes by synchronizing with the above wavelength selecting operation.
Therefore, an external resonator is formed by an optical path from the reflecting mirror 7 up to the semiconductor laser 5 through the diffraction grating 6 and it is possible to extraordinarily increase the intensity of the light having the designated specific wavelength λ2 compared to the intensity of light having another wavelength.
That is, the wavelength variable light source 1 makes it possible to optionally control the wavelength λ of the light “a” serving as the light output from the wavelength variable light source 1 output from the other side of the semiconductor laser 5.
In this case, a rotation amount (rotation angle) from a reference position of the motor 10 corresponds to the wavelength λ of the light “a” emitted from the wavelength variable light source 1 one to one.
Therefore, only by designating the wavelength λ to the motor driving circuit 11 from the measurement control section 4, the light “a” having the wavelength λ is output from the wavelength variable light source 1.
FIG. 12 is a flowchart shown to explain a procedure for measuring the wavelength characteristic of the measuring object 2 by the measurement control section 4.
That is, the measurement control section 4 measures the wavelength characteristic of the measuring object 2 in accordance with the flowchart shown in FIG. 12.
First, the measurement control section 4 sets measuring wavelengths λ1, λ2, . . . , and λN for the measuring object 2 (step S1).
Then, the measurement control section 4 initializes a measuring wavelength λn (n=1).
The measurement control section 4 designates the measuring wavelength λn to the wavelength variable light source 1 (step S3).
Then, when receiving a notice showing that setting of the measuring wavelength λn is completed from the wavelength variable light source 1 (step S4), the measurement control section 4 transmits a measurement command to the optical power meter 3 (step S5).
When receiving a measured value cn of the wavelength λn from the optical power meter 3 (step S6), the measurement control section 4 processes data for the received measured value cn (step S7).
When measurement of the measured value cn of optical power of one measuring wavelength λn is completed, the measurement control section 4 increments the measuring wavelength λn in step S8 (n=n+1).
Then, the measurement control section 4 returns to step S3 to start measurement of the optical power of the incremented measuring wavelength λn.
Then, when the measuring wavelength λn exceeds the final measuring wavelength λN in step S9, the measurement control section 4 completes the measuring of the measuring object 2 and edits and outputs the wavelength characteristic shown in FIG. 11.
However, the conventional wavelength-measuring apparatus provided with the configuration and functions shown in FIGS. 9 to 12 has the following problems to be solved yet.
That is, in the case of the wavelength variable light source 1 used for the conventional wavelength characteristic measuring apparatus, the wavelength λ of the emitted light “a” is measured by mechanically moving the position and attitude angle of the reflecting mirror 7 by the motor 10.
Therefore, in this case, it is necessary to confirm that the reflecting mirror 7 is moved to the intended position and attitude angle in accordance with the past record of the rotation angle of the motor 10.
Therefore, the measurement control section 4 must designate and confirm the measuring wavelength λn for the wavelength variable light source 1 by software whenever the measuring wavelength λn is changed.
In this case, accurate position control requires a lot of time due to a phenomenon peculiar to a motor such as backlash.
Moreover, the measurement control section 4 must capture the measurement command and measured value cn for the optical power meter 3 each time.
Thus, in the case of the conventional wavelength characteristic measuring apparatus, it takes a lot of time (e.g. 500 ms/pts) to measure the wavelength characteristic shown in FIG. 11 for one measuring object 2 due to the backlash of the above motor 10 or the like.
Moreover, in the case of the conventional wavelength characteristic measuring apparatus, the optical power meter 3 may be constituted by a light receiving section 12 and a variable amplifier 13 as shown in FIG. 13 in order to improve the measuring accuracy of the light intensity (light power) of the light b emitted from the measuring object 2.
That is, in the case of the conventional wavelength characteristic measuring apparatus, a signal e of the light b converted into an electrical signal output from the light receiving section 12 changes at a very wide level as shown in FIG. 13.
When amplifying the signal e by a normal amplifier having a fixed amplification factor, the linearity of an output is deteriorated because it is difficult to keep the output linearity in a wide input range in the case of the normal amplifier.
The above deterioration of the linearity of the amplifier affects the accuracy of light intensity measurement.
When converting an output of the amplifier for amplifying the signal e into a digital signal by an analog/digital (A/D) converter, the resolution of the converted digital signal is deteriorated if the input range of the A/D converter is increased.
To avoid the above problem, it is necessary to amplify a signal at an amplification factor α corresponding to the level of the signal by using the variable amplifier 13.
Setting of the amplification factor α in the variable amplifier 13 is executed by selecting one of a plurality of feedback circuits respectively constituted by a resistance 15 and a switch 16 which are connected to input and output terminals of an operational amplifier 14 in parallel from the outside.
Therefore, the conventional wavelength characteristic measuring apparatus separately requires a control circuit for detecting an amplified signal value output from the variable amplifier 13 and selecting an optimum feedback circuit so that the amplified signal value becomes an optimum level.
Further, a predetermined time is required by the time the amplification factor α is fixed to a predetermined value after feedback circuits to be connected to the operational amplifier 14 are changed. However, because measurement cannot be executed during the above period, the measuring efficiency is further deteriorated in the case of the conventional wavelength characteristic measuring apparatus.